The reliability, durability, and functional integrity of electrical components can be inversely related to the operating temperatures experienced in such devices, whether the heat is generated by the device itself or from other sources. Semiconductor technology can be characterized as a quest to place more electronic components in less space to achieve greater speed and performance. As integrated circuits and other semiconductor devices become faster, operating frequencies (e.g., clock speed in a microprocessor) also increase. At the same time, the distances between the conductive lines within the semiconductor device are reduced due to efforts to construct semiconductor devices that are increasingly compact.
As the density of conductive lines and the clock speed of circuits increase, the amount of heat generated by the device also increases. Therefore, it is critical to have an efficient heat-removal system associated with integrated circuits. One method to remove heat from an electronic assembly or an integrated semiconductor package assembly is to place a heat dissipating device made of heat-conducting material in thermal communication with a heat-generating component (or another heat dissipating device) to draw heat away from a heat-sensitive electronic component. For instance, a heat spreader to absorb heat from a heat generating device can be used in combination with a second level solution, such as a heat sink, a heat pipe, or a fan/heat sink device.
Thermal conductivity between proximate surfaces is related to the surface area that is in actual contact. Because surfaces of heat dissipating devices and electrical components are not completely smooth, at least on a microscopic level, it is difficult, if not impossible, to achieve perfect contact between their surfaces. Because air is a poor thermal conductor, air pockets that may remain between the surfaces can inhibit the conduction of heat from one surface to another. To overcome the effects of air as a thermal insulator, and to conduct heat from a device that generates heat, such as a chip die, to a heat dissipating device (or from one heat dissipating device to another), thermal interface material (TIM) has been developed to fill the gaps between the proximate surfaces. Semiconductor chip packages can use a primary TIM layer (TIM1) to thermally couple a die and a heat spreader, and/or a secondary TIM layer (TIM2) to thermally couple the heat spreader and a heat sink.
The TIM technologies used for electronic packages encompass several classes of materials such as phase change materials, epoxies, greases, and gels. However, such materials have only a moderate thermal conductivity and thus provide an inadequate level of heat removal or redistribution for many applications, such as high performance, high power processors. Other limitations of such TIM layers include the uncontrolled flow of the TIM when heated at operating temperatures, for example, into a bleed hole of a heat spreader. In addition, such TIM layers adhere to the surfaces of the components after disassembly, which requires cleanup in a testing environment, and reapplication after performing maintenance on a finished product. Furthermore, application of such TIM layers applied at the point of final assembly of an integrated semiconductor package is particularly undesirable when the die and the heat dissipating device are fabricated at remote locations.
Use of a metallic solder as the TIM layer can result in improved thermal conductivity. However, metallic TIM layers and processes for their application to the component surfaces have many drawbacks, including some of the aforementioned limitations. For example, achieving a durable intermetallic bond in the soldering process may require the reflow of the TIM, which may include heating a solid TIM preform located between two components. Heating at temperatures that potentially damage heat sensitive components may be required, however, and components having sufficiently different coefficients of thermal expansion may produce an unreliable intermetallic bond. Another impediment to achieving a reliable bond in the soldering process is that metal surfaces of the components are readily subject to oxidation. Accordingly, a chemical solder flux may be used in the soldering process. Use of a solder flux in soldering the TIM layer may result in permanent bonding of surfaces of each of the coupled components. A permanent attachment at both surfaces, however, makes subsequent disassembly of a final product difficult, and may be even more undesirable when using a TIM layer to thermally couple a heat dissipating device and a test vehicle.